The present invention relates to a refrigerating and air-conditioning apparatus using a non-azeotropic refrigerant, and more particularly to a refrigerating and air-conditioning apparatus which is capable of operating with high reliability and efficiency by accurately detecting the composition of the refrigerant which circulates in a refrigeration cycle using a non-azeotropic refrigerant in which three or more kinds of refrigerants are mixed.
First, a description will be given of the characteristics of the composition of a refrigerant which is circulated in a cycle of a refrigerating and air-conditioning apparatus using a non-azeotropic refrigerant. FIG. 15 is a gas-liquid equilibrium diagram illustrating the characteristics of a non-azeotropic refrigerant in which two kinds of refrigerants are mixed, and the ordinate represents the temperature, while the abscissa represents a circulating composition (a composition ratio of low-boiling components), the parameter being the pressure. In the case of the two-kinds-mixed non-azeotropic refrigerant, a saturated vapor curve and a saturated liquid curve are determined by the pressure, as shown in FIG. 15. The region located upwardly of the saturated vapor curve indicates a superheated vapor state, the region located downwardly of the saturated liquid curve indicates a supercooled state, and the region located between the saturated vapor curve and the saturated liquid curve indicates a gas-liquid two-phase state. In FIG. 15, Z denotes the circulating composition in a refrigeration cycle; a point 1, an outlet portion of a compressor; a point 2, an outlet portion of a condenser; a point 3, an inlet portion of an evaporator; and a point 4, an inlet portion of the compressor.
In general, in a refrigeration cycle using a non-azeotropic refrigerant, the composition of the refrigerant circulating in the cycle and the composition of the refrigerant charged in the cycle does not necessarily agree with each other. This is because, in the gas-liquid two-phase portion of the refrigeration cycle, which is shown by a point A in FIG. 15, the liquid composition becomes X which is smaller than the circulating composition Z, while the vapor composition becomes Y which is larger than the circulating composition. Particularly in a cycle in which an accumulator is provided in the piping between the outlet of the evaporator and the inlet of the compressor, if the liquid refrigerant accumulates in the accumulator, the circulating composition shows a tendency in which low-boiling components increase more than in the case of the charged composition. This is attributable to the fact that the liquid refrigerant, in which the amount of low-boiling components is smaller (the amount of high-boiling components is larger) than in the case of the charged composition, is accumulated in the accumulator.
In addition, even if the refrigerant in the refrigeration cycle has leaked outside, the circulating compositions in the cycle change. For example, if the leakage of the liquid refrigerant occurs in the gas-liquid two-phase portion shown by the point A in FIG. 15, the refrigerant with a composition X smaller than the circulating composition leaks, so that the circulating composition shows a tendency of becoming larger. On the other hand, if the vapor refrigerant leaks in the gas-liquid two-phase portion, the refrigerant with the composition Y larger than the circulating composition, so that the circulating composition shows a tendency of becoming smaller. Thus, in the cycle using the non-azeotropic refrigerant, the composition of the refrigerant circulating in the cycle changes substantially due to the operating condition of the cycle, the leakage of the refrigerant, and the like.
If the circulating composition in the cycle changes, the relationship between the pressure and the saturation temperature of the refrigerant changes, as can be seen from FIG. 15, and the cooling capabilities also change substantially. Accordingly, to make the cycle stable and allow predetermined capabilities to be demonstrated, it is necessary to accurately detect the circulating composition in the cycle, and optimally control the number of revolutions of the compressor, the amount of opening of a pressure reducing device, and the like in correspondence with the circulating composition.
FIG. 16 shows a configuration of a conventional refrigerating and air-conditioning apparatus using a non-azeotropic refrigerant, which is disclosed in, for example, Japanese Patent Application Publication. In the drawing, reference numeral 1 denotes a compress or; 2, a c on denser; 33, a receiver; 33 a pressure reducing device; and 4, an evaporator, and these component elements are consecutively connected by pipes and constitute a refrigeration cycle. As a refrigerant, a non-azeotropic refrigerant in which two kinds of refrigerants, including a high-boiling component and a low-boiling component, are mixed is used. In addition, a temperature detector 34 and a pressure detector 35 are provided for a receiver 33 at an outlet of the condenser 2, and signals from these detectors are inputted to a composition calculator 10.
With the conventional refrigerating and air-conditioning apparatus using a non-azeotropic refrigerant, which is figured as described above, the vapor of the high-temperature, high-pressure non-azeotropic refrigerant which is compressed by the compressor 1 is condensed and liquefied by the condenser 2, and flows into the receiver 33. This liquid refrigerant is passed through the pressure reducing device 3, where it is converted to a low-temperature, low-pressure gas-liquid two-phase refrigerant, and flows into the evaporator 4, where it is evaporated and returns to the compressor 1. The circulating composition in the cycle is calculated by the composition calculator 10 on the basis of information on the temperature and pressure of the liquid refrigerant which has flown into the receiver 33, the temperature and pressure having been detected by the temperature detector 34 and the pressure detector 35. That is, a gas-liquid equilibrium diagram such as the one shown in FIG. 17 is obtained from the kind of the charged non-azeotropic refrigerant in which two kinds of refrigerants are mixed, as well as the pressure PH detected by the pressure detector 35. If it is assumed that the state of the refrigerant in the receiver 33 is that of a saturated liquid, the circulating composition Z in the cycle can be detected from a point of intersection of the saturated liquid curve and the temperature TH detected by the temperature detector 34, as shown in FIG. 17.
If this basic principle of detection of the circulating composition is expanded, in the case of the two-kinds-mixed non-azeotropic refrigerant, the circulating composition can be detected if the quality of wet vapor, X, (=the flow rate of the mass of refrigerant vapor/the flow rate of the total mass of the refrigerant), as well as the temperature and pressure of the refrigerant at this quality of wet vapor, X, are known. That is, in the case of the two-kinds-mixed non-azeotropic refrigerant, in a case where the pressure P is fixed, the relationship such as the one shown by the dot-dashed line in FIG. 18, which illustrates the conventional basic principle of detection of circulating compositions of the two-kinds-mixed refrigerant, is present between the temperature and the circulating composition Z of the refrigerant at the quality of wet vapor, X, including the saturated vapor curve where the quality of wet vapor, X, =1 and the saturated liquid line where the quality of wet vapor, X, =0. Accordingly, by using this relationship, it is possible to detect the circulating composition in the cycle if the pressure, temperature, and quality of wet vapor of the refrigerant in the gas-liquid two-phase state, including the saturated vapor and the saturated liquid, can be known.
However, although this method can be applied to the two-kinds-mixed refrigerant in which two kinds of refrigerants are mixed, it cannot be applied to a mixed refrigerant in which three or more kinds of refrigerants are mixed. In the case of the two-kinds-mixed refrigerant, if the composition Z1 of the first component can be known, the composition Z2 of the second component can be determined as being (1-Z1). In the case of the three-kinds-mixed refrigerant, on the other hand, even if the composition Z1 of the first component alone can be known, there are infinite combinations of the composition Z2 of the second component and the composition Z3 of the third component, so that it is impossible to determine the overall composition.
A description will be given of this aspect with reference to the gas-liquid equilibrium diagram of a three-kinds-mixed refrigerant shown in FIG. 19. FIG. 19 is a gas-liquid equilibrium diagram of a three-kinds-mixed refrigerant under the conditions where the pressure P is fixed and the temperature T is fixed. The abscissa shows the composition Z1 of the first component, while the ordinate shows the composition Z2 of the second component. The two solid lines in the drawing show the saturated vapor curve and the saturated liquid curve. The region located upwardly of the saturated vapor curve indicates a superheated vapor state, the region located downwardly of the saturated liquid curve indicates a supercooled state, and the region surrounded by the saturated vapor curve and the saturated liquid curve indicates a gas-liquid two-phase state. The dot-dashed line in the drawing indicates the state in which the quality of wet vapor, X, is fixed in the gas-liquid two-phase state. As is apparent from this drawing, in the case of the three-kinds-mixed refrigerant, even if the pressure P, temperature T, and quality of wet vapor, X, of the refrigerant in the gas-liquid two-phase state are known, it can only be known that the circulating composition is present on the dot-dashed line in the drawing, and it is impossible to determine the circulating composition, i.e., the composition of the first component and the composition of the second component of the circulating composition. Incidentally, in the case of the three-kinds-mixed refrigerant, if the composition Z1 of the first component and the composition Z2 of the second component are known, the composition Z3 of the remaining third component is determined uniquely from (1-Z1-Z2).
As a conventional method of detecting circulating compositions of the three-kinds-mixed non-azeotropic refrigerant, a method disclosed in, for example, Japanese Patent Application Laid-Open No. 261576/1996 is known. FIG. 20 is a schematic diagram of a conventional refrigerating and air-conditioning apparatus using a three-kinds-mixed non-azeotropic refrigerant. In the drawing, reference numeral 1 denotes a compressor; 2, a condenser; 3, a pressure reducing device; 4, an evaporator; and 5, an accumulator, and these component elements are consecutively connected by pipes and constitute a refrigeration cycle. As a refrigerant, a non-azeotropic refrigerant in which three kinds of refrigerants having different boiling points are mixed is used. In addition, numeral 41 denotes a bypass pipe provided between an outlet of the condenser 2 and the accumulator 5, and a capillary tube 42 is provided midway in the pipe. Further, a temperature detector 43 and a pressure detector 46 are provided at a suction pipe of the compressor 1, and temperature detectors 44 and 45 are respectively provided before and after the capillary tube 42 of the bypass pipe 41. Signals from these three temperature detectors 43, 44, and 45 and the pressure detector 46 are inputted to a composition calculator 10.
A description will be given of the basic principle of detection of the circulating composition in the conventional refrigerating and air-conditioning apparatus configured as described above and using a non-azeotropic refrigerant. The interior of the accumulator 5 is in a saturated state at the pressure P1, and the saturated vapor of the three-kinds-mixed refrigerant having a composition of y1, y2, and y3 is present in an upper portion thereof, while the saturated liquid having a composition of x1 , x2, and x3 is present in a lower portion thereof. The circulating composition in the cycle is identical to y1, y2, and y3, and this circulating composition is calculated on the basis of the signals from the three temperature detectors 43, 44, and 45 and one pressure detector 46. First, the temperature T1 and the pressure P1 within the accumulator 6 are detected by the temperature detector 43 and the pressure detector 46. The saturated vapor composition at the temperature T1 and the pressure P1 is on the saturated vapor curve shown by the solid line in the gas-liquid equilibrium diagram of the tree-kinds-mixed refrigerant in FIG. 21, and it can be appreciated that the circulating composition is also present on the saturated vapor curve.
Next, the inlet temperature T2 and the outlet temperature T3 of the capillary tube 42 of the bypass pipe 41 are detected by the temperature detectors 44 and 45. In the capillary tube portion, the refrigerant undergoes an isenthalpic change, the enthalpy before and the enthalpy after the capillary tube 42 are equal, so that the enthalpy of this portion can be known from the inlet temperature T2 of the capillary tube 42. Accordingly, the temperature, pressure, and enthalpy of the gas-liquid two-phase refrigerant at the outlet portion of the capillary tube 42 become known amounts, so that the quality of wet vapor, X3, can be determined. That is, the temperature T3, the pressure P1, and the quality of wet vapor, X3, of the gas-liquid two-phase refrigerant at the outlet portion of the capillary tube become known amounts. In the gas-liquid equilibrium diagram of the tree-kinds-mixed refrigerant shown in FIG. 21, the composition of the gas-liquid two-phase refrigerant at the quality of wet vapor, X3, at the temperature T3 and the pressure P1 is represented by the broken line, and it can be appreciated that the circulating composition is also present on this broken line.
From the above, it can be seen that the circulating composition is present on the saturated vapor curve at the temperature T1 and the pressure P1 shown in FIG. 21, and is present on the fixed line of the quality of wet vapor, X3, at the temperature T3 and the pressure P1, shown in FIG. 22 which is a gas-liquid equilibrium diagram of the tree-kinds-mixed refrigerant. Accordingly, as shown in FIG. 23 which is a diagram illustrating the basic principle of detection of circulating compositions of a three-kinds-mixed refrigerant, if these two diagrams are superposed on top of the other, the first component y1 and the second component y2 of the circulating composition can be determined as a point of intersection of these two curves, while the third component y3 can be determined as being (1-y1-y2), thereby making it possible to determine the circulating composition of y1, y2, and y3.
However, with such a method of detecting the circulating composition, three temperature detectors and one pressure detector are required. In addition, as can be seen from FIG. 23, the difference between the gradient of the saturated vapor curve at the temperature T1 and the pressure P1 and the gradient of the fixed line of the quality of wet vapor, X3, at the temperature T3 and the pressure P1 is very small in terms of the basic principle. Hence, to determine the circulating composition as the point of intersection of these two curves, it is necessary to accurately identify the two curves. Namely, to accurately identify these two curves, three temperature detectors and one pressure detector which are highly accurate are required, so that there has been a drawback in that the apparatus becomes expensive. In addition, if temperature detectors and a pressure detector of the accuracy used in an ordinary refrigerating and air-conditioning apparatus are used, there has been a drawback in that errors of the two curves become large, with the result that a large error is included in the circulating composition which is determined as the point of intersection, making it impossible to operate the refrigerating and air-conditioning apparatus stably and with high reliability.
With the conventional refrigerating and air-conditioning apparatuses using a non-azeotropic refrigerant, most apparatuses are able to detect the circulating compositions of two-kinds-mixed refrigerants but are unable to detect the circulating compositions of mixed refrigerants in which three or more kinds of refrigerants are mixed. In addition, although some apparatuses have been proposed which are able to detect the circulating compositions of three-kinds-mixed refrigerants, the number of sensors required is large, and high accuracy is required of the sensors, so that there has been a drawback in that the apparatuses become expensive. In addition, in a case where temperature detectors and a pressure detector of the accuracy used in an ordinary refrigerating and air-conditioning apparatus are used, there has been a drawback in that errors of the two curves become large, with the result that a large error is included in the circulating composition which is determined as the point of intersection, making it impossible to operate the refrigerating and air-conditioning apparatus stably and with high reliability.